The top U.S. military officer confirmed Thursday that Islamic State militants targeted a military base in Iraq where U.S. troops were stationed with a potentially deadly chemical weapon this week.

“We assess it to be a sulfur-mustard blister agent,” Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, told the Senate Armed Services Committee.

Dunford did not elaborate, but the shell landed at a military base in northern Iraq where U.S. military advisers are helping Iraqi forces prepare for an upcoming offensive, according to an earlier account from two defense officials. They asked not to be named because they were not authorize to discuss the issue publicly. The defense officials suspected the Islamic State, also known as ISIL or ISIS, of launching the attack.

The shell landed in Qayara West, an air base that was seized from the Islamic State recently and is serving as a staging area for the upcoming offensive to retake Mosul, Iraq’s second largest city, from the militants, the officials said.

No one was killed or injured in the Tuesday attack. The defense officials said American troops were several hundred yards from where the shell landed.

Dunford said U.S. troops have adequate protection against chemical weapons and have detection and decontamination equipment.

“It wasn’t particularly effective but it was a concerning development,” Dunford said.

Dunford said the coalition has conducted about 30 airstrikes on the Islamic State's “emerging” chemical capabilities. Earlier this month the Pentagon said it struck a pharmaceutical factory in Mosul that had been converted by the Islamic State into a chemical weapons factory.

The U.S. government has previously accused the Islamic State of using chemical weapons.